Salt-splitting electrolysis system comprising flow electrodes and methods of operating such systems

ABSTRACT

Described herein are salt-splitting electrolysis systems, which comprise flow electrodes, and methods of operating such systems. Specifically, the flow electrodes comprise active particles (suspended in a solvent) with catalysts. These catalysts are configured to react with either cations or anions, provided in a feed stream. The flow electrodes allow using the same system for different feed streams, e.g., by flowing different types of electrodes through the system. Furthermore, the flow electrodes allow in-situ catalyst reconditioning. For example, the active particles can be flown from the current collectors to respective recovery devices where the particles are discharged or subjected to a reverse potential. The active particles can be conductive and provide more desirable electrical field distribution between the current collectors resulting in greater ionic mobility. Finally, the active particles concentrate ions around the particles thereby providing a higher concentration gradient through separating structures, which enclose the feed stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) of US Provisional Pat. App. 63/266,026, filed on Dec. 27, 2021, which is incorporated herein by reference in its entirety for all purposes.

BACKGROUND

Many chemicals (e.g., caustic soda, sulfuric acid, metal salts for lithium-ion batteries) are produced and refined through chemical processes that cause major carbon-dioxide emissions. For example, the current manufacturing of lithium hydroxide processing uses lithium brine concentrated in evaporation ponds with a lithium content of 50-500 ppm. The brine is purified, filtered, and treated with sodium carbonate to precipitate the lithium as lithium carbonate. Lithium carbonate is then reacted with slaked lime to obtain lithium hydroxide. However, this process requires high purity of starting components and can have low yields/ high lithium-ion losses. New processes with fewer processing steps, lower lithium losses, lower initial cost, and reduced chemical reagent consumption are needed. Electrolysis processes provide such opportunities. Furthermore, electrolysis processes, if relying on renewable sources of energy, can dramatically reduce the carbon footprints associated with base chemicals manufacturing.

Salt splitting or, more specifically, salt-splitting electrolysis is a promising new technology used for decomposing various salts, e.g., to recover valuable components. Conventional salt-splitting systems use current collectors coated with catalysts, collectively operable as electrodes. One, two, or multiple stacks of membranes can be used for ionic separation between the two electrodes. Once ions reach their respective electrodes, the ions undergo target reactions converting these ions into desirable productions. However, electrodes tend to have a limited lifetime due to surface oxidation and passivation, loss of catalytic coating, and need to be periodically replaced and/or reconditioned. This electrode replacement is one of the largest costs associated with operating conventional salt-splitting systems, limiting the use of electrolytic processes to a small group of commodity chemicals.

Furthermore, ionic transfer through membranes can be limited due to the high distance of separation between the membranes and the electrodes which can limit the strength/permeation of the electrical field driving ions through the membranes.

In salt-splitting electrolysis, maximizing the salt concentration during electrolysis is critical for reducing the costs associated with post-treatment processing such as evaporation and crystallization as well as reducing the operating voltage of the electrolysis process itself. Currently, operating at elevated temperatures (50-90C) is the only way to increase the solubility of salts in the electrolyte and reduce the operating voltage of the electrolysis process. However, elevated temperatures also result in accelerated corrosion and dissolution of the catalyst materials into the electrolyte, especially under anodic potentials (>1 V) and high current densities (1000-6000 A/m2).

What is needed are new salt-splitting systems that address the use of expensive catalysts that undergo electrode passivation and dissolution. In addition, what is needed are systems that can significantly improve the ionic transfer limitation of the conventional systems and allow operation at much higher salt concentrations (such as at least about 50 g/L, or even at least about 150 g/L).

SUMMARY

Described herein are salt-splitting electrolysis systems, which comprise flow electrodes, and methods of operating such systems. Specifically, the flow electrodes comprise active particles (suspended in a solvent) with catalysts. These catalysts are configured to react with water molecules, as well as cations or anions, provided in a feed stream. The flow electrodes allow using the same system for different feed streams, e.g., by flowing/filling different types of electrode particles through the system. Furthermore, the flow electrodes allow in-situ catalyst reconditioning. For example, the active particles can be flown from the current collectors to respective recovery devices where the particles are discharged or subjected to a reverse potential. The active particles can be conductive and provide more desirable electrical field distribution between the current collectors and the membranes resulting in greater ionic mobility and higher ion transfer rates. Finally, the active particles concentrate ions around the particles capacitively thereby providing a higher concentration gradient through separating structures, which enclose the feed stream. This concentration gradient also helps keep most of the salt ions at the double-layer surface of the particles, lowering the effective local concentration of the electrolyte. In turn, higher salt concentrations above the practical limitations of salt solubilities can be achieved in the electrolytes, allowing higher electrolyte conductivities during operation for lower operating voltage and reduced post-processing costs.

In some examples, a salt-splitting electrolysis system comprises a feed stream, comprising a solvent and a salt, dissolved in the solvent and comprising cations and anions. The system also comprises a negative current collector and a negative flow electrode, comprising the solvent and negative active particles, suspended in the solvent and comprising a negative catalyst configured to react with the cations. The system also comprises a negative separating structure, disposed between and in contact with the feed stream and the negative flow electrode. The negative separating structure is configured to selectively pass the cations from the feed stream to the negative flow electrode. The system comprises a positive current collector and a positive flow electrode, comprising the solvent and positive active particles, suspended in the solvent and comprising a positive catalyst configured to react with the anions. Furthermore, the system comprises a positive separating structure, disposed between and in contact with the feed stream and the positive flow electrode. The positive separating structure is configured to selectively pass the anions from the feed stream to the positive flow electrode.

In some examples, each of the negative active particles comprises a negative base structure, comprising supporting the negative catalyst. In the same or other examples, each of the positive active particles comprises a positive base structure, comprising supporting the positive catalyst. For example, at least one of the negative base structure and the positive base structure comprises an electronically conductive material. The electronically conductive material may be one or more of graphitic carbon, glassy carbon, graphene, carbon nanotubes, transition metal carbides (e.g., Ti₃C₂), and other catalytic metal particles such as iron oxide, platinum, and, iridium.

In some examples, the negative catalyst comprises one or more of graphitic carbon, carbon-nitride, graphene, carbon nanotubes, transition metal carbides, platinum, palladium, rhodium, and iridium. In the same or other examples, the positive catalyst comprises one or more of graphitic carbon, carbon-nitride, graphene, carbon nanotubes, transition metal carbides (e.g., Ti₃C₂), platinum, palladium, rhodium, iridium, metal oxides (e.g., iron oxides, nickel oxides, cobalt oxides, manganese oxides), and perovskite oxides (with the formula ABO3, e.g., Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O₃ and (Ln_(0.5)Ba_(0.5))CoO₃).

In some examples, the negative separating structure is one of a cation-exchange membrane or a first size-exclusion porous separator. The positive separating structure is one of an anion-exchange membrane or a second size-exclusion porous separator.

Also provided is a method of operating a salt-splitting electrolysis system. The method comprises flowing a feed stream into a feed channel of the salt-splitting electrolysis system, such that the feed stream comprises cations and anions. The cations are selectively transferred to a negative flow electrode comprising negative active particles. The anions are selectively transferred to a positive flow electrode comprising positive active particles. The negative active particles comprise a negative catalyst configured to react with the cations in the feed stream. The positive active particles comprise a positive catalyst configured to react with the anions. The method also comprises applying a voltage between a negative current collector and a positive current collector of the salt-splitting electrolysis system thereby causing the cations in the feed stream to react with the negative catalyst and further causing the anions in the feed stream to react with the positive catalyst.

In some examples, the negative active particles are suspended in a solvent and flow through a negative chamber of the salt-splitting electrolysis system. The positive active particles are suspended in the solvent and flow through a positive chamber of the salt-splitting electrolysis system. More specifically, the negative active particles are uniformly distributed through the negative flow electrode. The positive active particles are uniformly distributed through the positive flow electrode.

In some examples, the negative active particles are contained within a set portion of a negative chamber of the salt-splitting electrolysis system such that a remaining portion of the negative flow electrode flows through the negative active particles. The positive active particles are contained within a set portion of a positive chamber of the salt-splitting electrolysis system such that a remaining portion of the positive flow electrode flows through the positive active particles.

In some examples, the method further comprises pumping at least a portion of the negative flow electrode into a negative electrode recovery device, recovering at least the portion of the negative flow electrode using the negative electrode recovery device, and pumping at least the portion of the negative flow electrode back to the negative current collector. The method also comprises pumping at least a portion of the positive flow electrode into a positive electrode recovery device, recovering at least the portion of the positive flow electrode using the positive electrode recovery device, and pumping at least the portion of the positive flow electrode back to the positive current collector.

In some examples, the method comprises recovering at least the portion of the negative flow electrode using the negative electrode recovery device comprises discharging the negative active particles of the negative flow electrode. The method also comprises recovering at least the portion of the positive flow electrode using the positive electrode recovery device comprises discharging the positive active particles of the positive flow electrode.

In some examples, the method comprises discharging the negative active particles of the negative flow electrode comprises flowing the negative active particles past a ground connection within the negative electrode recovery device or applying a reverse potential to the negative flow electrode. The method also comprises discharging the positive active particles of the positive flow electrode comprises flowing the positive active particles past a ground connection within the positive electrode recovery device or applying a reverse potential to the positive flow electrode.

In some examples, the voltage applied between the negative current collector and the positive current collector is between 1.5 V and 10 V per cell. In some examples, applying the voltage between the negative current collector and the positive current collector causes a current density between 2000 A/m² and 8000 A/m² (based on the membrane area) between the negative current collector and the positive current collector. Furthermore, in some examples, the method comprises pretreating the feed stream before flowing the feed stream into the feed channel. For example, pretreating the feed stream comprises flowing the feed stream through one or more ion-exchange columns, configured to selectively capture one or more of calcium (Ca), magnesium (Mg), iron (Fe), boron (B), sodium (Na), potassium (K), chloride (Cl), and sulfate (SO₄).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic representation of a salt-splitting electrolysis system comprising flow electrodes, in accordance with some examples.

FIG. 1B is another example of a salt-splitting electrolysis system comprising flow electrodes.

FIGS. 2A, 2B, and 2C are block diagrams of different components of the salt-splitting electrolysis systems in FIGS. 1A and 1B, in accordance with some examples.

FIGS. 3A and 3B are schematic representations of negative active particles and positive active particles, in accordance with some examples.

FIG. 4 is a process flowchart corresponding to a method of operating a salt-splitting electrolysis system, in accordance with some examples.

DETAILED DESCRIPTION

In the following description, numerous specific details are outlined to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail to avoid obscuring the present invention. While the invention will be described in conjunction with the specific examples, it will be understood that it is not intended to limit the invention to the examples.

Introduction

Salt-splitting electrolysis can be used for various applications. Some examples include, but are not limited to, (1) chlor-alkali production of caustic soda and hydrochloric acid, (2) processing lithium salts (e.g., Li₂SO₄, LiCl, LiOH) for the battery industry and production of sulfate and chloride acids (H2SO4 and HCl) as by-products, (3) processing potash (potassium) salts (e.g., K₂CO₃, KCI, K₂SO₄, KNO₃, K2O, KOH, etc.) for agricultural and industrial uses, (4) hydrometallurgical processing of critical minerals (e.g., cobalt, nickel, manganese) for the battery industry, and (5) recovering rare earth elements (REEs) including the light REEs (LREEs) such as lanthanum, cerium, praseodymium, neodymium, samarium, and europium, as well as the heavy REEs (HREEs) such as gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium.

Unfortunately, many conventional processes used for these applications have many drawbacks, such as toxic waste, high-energy consumption, and carbon emissions. In comparison to electrolysis, chemical conversion has disadvantages, such as a low degree of metal recovery (e.g., a portion of the metal ions of interest (e.g. lithium) gets trapped in pre-treatment steps while removing other impurities), high-energy and resource consumption (e.g., high amounts of heat and water are often necessary to do refining due to use of large tanks and elevated temperatures), and high carbon emissions (e.g., use of carbon-bullish chemicals such as soda ash, limestone, and caustic soda).

Salt-splitting electrolysis has been a promising alternative but has high capital and operating costs. For example, conventional salt-splitting electrolysis systems use mixed metal oxide (MMO) or noble metal catalyst coatings on the electrodes such as IrO₂ and RuO₂, which are very expensive and require periodic replacement and/or reconditioning due to stability issues at elevated temperatures (50-90° C.) and high current densities (1000-8000 A/m²). The solubility of the salt solutions in the electrolytes is also limited by the operating temperature. Furthermore, ion exchange membranes used in conventional salt-splitting electrolysis systems can also be expensive while limiting ionic flux (e.g., due to the electric field distribution within the systems).

Salt-splitting electrolysis systems, describe herein, utilize flow electrodes, which address many limitations of the conventional systems. More specifically, these flow-electrode systems help to increase the ionic flux / current density (per unit area) through the separating structures (e.g., membranes, separators) in comparison to conventional systems. As such, the overall process has a higher throughput and/or requires smaller equipment. Flow electrodes also provide an opportunity for new catalyst materials. For example, flow electrodes are suspensions of active particles in the solvent (e.g., water). These active particles comprise base structures and are functionalized with respective catalysts (e.g., a negative catalyst configured to react with target cations and a positive catalyst configured to react with target anions). In some examples, high-conductivity, high-surface-area carbon materials, and other metallic powders can be used as active particles. Lastly, the particles also help increase the practical operating concentration of the salts in the electrolytes (e.g., to at least about 50 g/L or even at least about 150 g/L), which lowers the operating voltage of the electrolysis cell (e.g., to below 3 V or even below 2 V at 4000 A/m2) and helps reduce costs for downstream processes such as evaporation and crystallization.

In some examples, salt-splitting electrolysis systems (described herein) utilize flow electrodes in which active particles are stationary while solvents flow part these active particles to bring reactive entities (to the active particles) and remove reacted products (from the active particles). This configuration can be also referred to as packed-bed electrodes, fluidized-bed electrodes, or stationary-particles electrodes, e.g., to differentiate from moving-particles electrodes. For example, the active particles are supported (rather than flown) within specifically configured filters in each of the respective chambers. It should be noted that both the stationary-particles electrodes and moving-particles electrodes are examples of flow electrodes. However, in stationary-particles electrodes, the active particles remain stationary adjacent to respective current collectors while the solvents are circulated between current collectors and electrode recovery devices. In moving-particle electrodes, both the active particles and the solvents are circulated between current collectors and electrode recovery devices.

The stationary-particle electrode configuration provides various benefits of moving-particles electrodes but reduces various complexities associated with flowing high-viscosity slurries (i.e., flowing both the active particles and the solvents). The viscosity of the solvents alone is substantially less than the viscosity of the slurries formed by combining the active particles and the solvents.

Both the stationary-particles electrode configuration and the moving-particles electrode configuration can utilize a 2- or 3-compartment cell design. However, a stationary-particle electrode system only pumps relevant solvents / brines. When the brines pass through the chambers, filled with catalyst-containing particles, various target reactions take place the product brine is flown from the system while the particles remain in the chamber at all times. One having ordinary skills in the art would understand how to apply various features described in the context of the moving-particles electrode configuration to the stationary-particles electrode configuration.

Examples of Salt-Splitting Electrolysis Systems

FIG. 1A is a schematic representation of salt-splitting electrolysis system 100, in accordance with some examples. Salt-splitting electrolysis system 100 comprises feed stream 140, which can flow through specially-configured feed-stream channel 149.

In some examples, feed stream 140 comprises solvent 141 and salt 142. Salt 142 is dissolved in solvent 141 and comprises cations 143 and anions 144, e.g., as schematically shown in FIG. 2A. Some examples of salt 142 and other like materials (e.g., hydroxides) include, but are not limited to, lithium carbonate (Li₂CO₃), lithium hydroxide (LiOH), lithium chloride (LiCI), lithium sulfate (Li₂SO₄), nickel sulfate (NiSO₄), cobalt sulfate (CoSO₄), manganese sulfate (MnSO₄), nickel hydroxide (NiOH₂), cobalt hydroxide (CoOH₂), manganese hydroxide (MnOH₂), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), soda ash (NaCO₃), caustic soda (NaOH), iron phosphate (FePO₄), potash salts (e.g., K₂CO₃, KCI, K₂SO₄, KNO₃, K2O, KOH, etc.), sulfate salts of rare earth metals (e.g., REEs (LREEs) lanthanum, cerium, praseodymium, neodymium, samarium, and europium, as well as the heavy REEs (HREEs) such as gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium). The concentration regimes for each of these salts change depending on their respective solubilities at processing temperatures (e.g., room temperature). In some examples, the salt concentration is between 0.25 M and 1 M at the inlet and/or between 1 M and 4 M at the outlet. For instance, for lithium sulfate, the solubility limit of aqueous solutions at elevated temperatures (80° C.) is around 2 M for conventional electrolysis applications. With the use of flow electrodes, the solubility limit of lithium sulfate can be increased up to 4 M without any precipitation. This is mainly due to the capacitive effect of the charged particles, which immobilizes dissolved salt ions at the double layer, allowing the bulk solubility of the salt to substantially increase.

Salt-splitting electrolysis system 100 comprises negative current collector 110. Some examples of materials suitable for negative current collector 110 include, but are not limited to, graphite, stainless steel, nickel, titanium, iridium oxide, ruthenium oxide, activated carbon, and transition metal carbides. The current collectors can be in various form factors including but not limited to sheets, plates, meshes, expanded forms, and porous structures. These materials need to be electronically conductive and inert to the electrode materials and electrolytes that come in contact with negative current collector 110.

Salt-splitting electrolysis system 100 comprises negative flow electrode 120. Referring to FIG. 1C, negative flow electrode 120 comprises solvent 141 and negative active particles 122, suspended in solvent 141. Referring to FIG. 3A, negative active particles 122 comprise negative catalyst 125 configured to react with cations 143. In some examples, negative catalyst 125 comprises one or more of graphitic carbon, carbon-nitride, graphene, carbon nanotubes, transition metal carbides, as well as oxide and/or metallic forms of platinum, titanium, nickel, silver, copper, vanadium, palladium, rhodium, and iridium. Furthermore, negative active particles 122 comprise negative base structure 126. In some examples, negative base structure 126 comprises an electronically conductive material, such as graphitic or glassy carbon, mesoporous carbide-derived carbons, transition metal carbides, and or metallic particles of common transition metals such as titanium, silver, platinum, nickel, copper, etc. In some examples, negative catalyst 125 is doped onto base structure 126 or otherwise attached to base structure 126. In specific examples, negative catalyst 125 is an organic material (e.g., thiosemicarbazones), or other metallic or oxide catalysts such as platinum, titanium, nickel, silver, copper, vanadium, palladium, rhodium, molybdenum disulfate, and iridium grafted into base structure 126. Depending on the density of the catalyst material, the catalyst loading on the particles can be between 0.5 g/m² up to 50 g/m² or between 1-200 mg/g. Particle sizes can range between 1-20 micrometers.

Salt-splitting electrolysis system 100 comprises negative separating structure 130, disposed between and in contact with feed stream 140 and negative flow electrode 120. Negative separating structure 130 is configured to selectively pass cations 143 from feed stream 140 to negative flow electrode 120. Some examples of negative separating structure 130 include but are not limited to ionomer-based polymeric proton exchange membranes such as NAFION® N-324, NAFION® N-424, NAFION® N-117, and the like.

Salt-splitting electrolysis system 100 comprises positive current collector 170. Some examples of materials suitable for positive current collector 170 include, but are not limited to graphite, stainless steel, nickel, titanium, iridium, ruthenium, silver, copper, activated carbon, and transition metal carbides. The current collectors can be in various form factors including but not limited to sheets, plates, meshes, expanded forms, and porous structures.

Salt-splitting electrolysis system 100 comprises positive flow electrode 160, comprising solvent 141 and positive active particles 162, suspended in solvent 141. Referring to FIG. 3B, positive active particles 162 comprise positive base structure 166 and positive catalyst 165, supported on positive base structure 166 and configured to react with anions 144. In some examples, positive catalyst 165 comprises one or more of stainless steel, nickel, cobalt, nickel alloys, steel alloys, graphitic carbon, carbon-nitride, graphene, carbon nanotubes, transition metal carbides (e.g., Ti₃C₂), platinum, palladium, rhodium, iridium, metal oxides s(e.g., iron oxides, nickel oxides, cobalt oxides, manganese oxides), and perovskite oxides (with the formula ABO3, e.g., Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O₃ and (Ln_(0.5)Ba_(0.5))CoO₃). Depending on the density of the catalyst material, the catalyst loading on the particles can be between 0.5 g/m2 up to 50 g/m2 or between 1-200 mg/g. Particle sizes can range between 1-20 um.

In some examples, positive base structure 166 comprises an electronically conductive material, such as graphitic or glassy carbon, mesoporous carbide-derived carbons, transition metal carbides, and other metallic materials such as stainless steel, nickel, titanium, iridium, ruthenium, silver, and copper. In some examples, positive catalyst 165 is doped onto positive base structure 166 or otherwise attached to positive base structure 166. In specific examples, positive catalyst 165 is an organic material, grafted into positive base structure 166.

Salt-splitting electrolysis system 100 also comprises positive separating structure 150, disposed between and in contact with feed stream 140 and positive flow electrode 160. Positive separating structure 150 is configured to selectively pass anions 144 from feed stream 140 to positive flow electrode 160. Some examples of positive separating structure 150 include, but are not limited to, ionomer-based polymeric anion exchange membranes such as Fumatech FAB, Selemion, and the like.

Referring to FIG. 1A, feed stream 140 (which may be also referred to as a brine solution) is pumped through feed channel 149 that is in contact with negative separating structure 130 (which may be also referred to as a cation exchange membrane) and positive separating structure 150 (which may be also referred to as an anion exchange membrane). The voltage applied between negative current collector 110 and positive current collector 170 facilitates the salt splitting/conversion process. Once the positive and negatively charged salt ions migrate across their respective separating structures/membranes, these ions are introduced into respective flow electrodes. The electrochemical reactions occur at the surface of the electroactive particles (provided in these flow electrodes). As noted above, electroactive particles comprise catalysts that promote the described reaction (e.g., the formation of hydroxide and/or hydrogen gas at the negative electrode and the formation of hydrogen ions and/or oxygen gas at the positive electrode in some examples).

FIG. 1B illustrates another example of salt-splitting electrolysis system 100. Unlike the example in FIG. 1A, salt-splitting electrolysis system 100 in FIG. 1B does not have feed-stream channel 149. In this example, the feed stream (e.g., a brine) is introduced into one of the flow electrodes. In this case, the system is operated under a “batch” mode configuration where the system accumulates charges and products over time. A net flow of positive ions from the negative compartment to the positive compartment occurs over time. Once concentrations close to the solubility limit with flow electrodes are reached, the solutions are replenished. In the case of lithium sulfate electrolysis, lithium ions move across the separating structure/membrane and get converted into LiOH at the positive electrode. In this process, the pH of the negative electrolyte cumulatively drops up to 75% conversion of lithium ions to protons.

There are several advantages to introducing flow-electrode technology into salt-splitting electrolyzer systems. First, the electroactive particles in the flow electrodes are electrically conductive. Hence, with the presence of the conductive particles, the effective reach of the applied electrical potential gets significantly amplified inside the electrolyzer cells. This way, the effective strength of the electric field (i.e. the pull of the electric field on the salt ions) gets enhanced, significantly increasing the ion flux per unit area across the membranes. Compared to conventional electrolysis systems where the electrode surface to reactant volume ratio is usually less than 1, more likely around 0.1. With the use of flow electrodes, the surface-to-volume ratio can be increased by multiples of up to 10. This significant increase in surface increase results in a multi-fold increase in the reaction kinetics as well.

Furthermore, when a potential difference is applied, the conductive particles absorb ions to their surface, forming a double layer. This way, local salt concentration around the particles and near the membrane surface decreases. As a result, the effective concentration gradient across the membrane can be significantly increased, resulting in increased ion transfer across the membranes and higher salt solubilities in the electrolytes for lower voltage operation and cost reductions in post-processing. At room temperature, the solubility of NaCl is around 35 g/L. With the use of carbon-based flow electrodes, concentrations of NaCl in solution up to 120 g/L can be achieved. This is a significant increase in the solubility of salts in water that is particularly beneficial for salt-splitting electrolysis.

The integration of catalyst materials into electroactive particles (instead of positioning on stationary current collectors) increases the effective reaction area (available for the salt conversion process). Compared to conventional electrolysis systems where the electrode surface to reactant volume ratio is usually less than 1, more likely around 0.1. With the use of flow electrodes, the surface-to-volume ratio can be increased by multiple folds up to 10. This significant increase in surface increase results in a multi-fold increase in the reaction kinetics as well. This aspect also increases the overall conversion efficiency and conversion rate of the process. Moreover, the use of catalyst materials on flowable electroactive particles enables the decoupling of the target electrochemical reactions from the material used for the fabrication of various internal components of the electrolyzer. In other words, the same electrolyzer cell stack can be used for multiple different electrolysis processes simply by changing the formulation and chemistry of the electroactive catalyst flow electrodes. For instance, without changing any of the separating structures, the same system can be utilized for both lithium sulfate and sodium sulfate electrolysis. Simply by changing the loading of the flow electrode particles in the solution in accordance with the solubility limits of Li and Na, two different salts can be split using the same electrolysis architecture.

Various formulations of the flow electrodes and various chemical and/or heat treatments of the electroactive particles are within the scope. In some examples, commercially-available catalyst formulations can be used to dope into the carbon and other metallic particles. In other examples, organic-based catalyst molecules (e.g., thiosemicarbazones) and other catalyst coatings are grafted onto the carbon particles to promote target reactions. The use of organic catalysts can be useful in moving away from expensive precious metal catalysts however might have issues with long-term stability and robustness. On the other hand, the use of precious metals can be expensive but allow stable, long-term operation of the particles.

Finally, the combination of conductive flow electrodes and high-surface-area catalyst materials allows various salt-splitting processes to happen at higher charge transfer rates with lower voltages/overpotentials. For instance, at 4000 A/m² current density, the voltage can be as much as 40% less compared to conventional electrolysis systems. In addition, due to the significant increase in the surface-to-volume ratio, the reactor active area necessary for the same output can be as low as 80% less.

Examples of Methods for Operating Salt-Splitting Electrolysis Systems

FIG. 4 is a process flowchart corresponding to method 400 of operating salt-splitting electrolysis system 100, in accordance with some examples. Various aspects of salt-splitting electrolysis system 100 are described above with reference to FIGS. 1A and 1B.

Method 400 comprises (block 410) pretreating feed stream 140. Various examples of feed stream 140 are described above with reference to FIG. 2A. The quality of the product made by the electrolysis process is determined by the purity of the metal salt produced. In this manner, any critical impurity in feed stream 140 (e.g., incoming brine) is removed through a pretreatment step to adjust the composition of feed stream 140 (e.g., to achieve high-purity salt conversion). The pretreatment operation depends on the application and the type of feed stream 140. For example, ion-exchange columns can be used to selectively capture certain impurity ions, such as calcium (Ca), magnesium (Mg), iron (Fe), boron (B), sodium (Na), potassium (K), chloride (Cl), and sulfate (SO₄). Especially the concentration of divalent ions such as Ca and Mg should be less than 100 ppb, chlorides (CI—) should also be less than 1 ppm, Na and K are less significant as they only affect the efficiency of the system but do not cause any harm to the membranes or components.

Method 400 proceeds with (block 420) flowing feed stream 140 into feed channel 149 of salt-splitting electrolysis system 100. For example, salt-splitting electrolysis system 100 can be equipped with a feed-stream pump for performing this flowing operation. Flow rates depend on the size of the reactor and the parallel/series configuration of the cell stack. In some examples, the flow rate (per membrane area) can be between 1 and 50 liters/minute-m². For example, the flow rate of flowing feed stream 140 can be on the higher end of this range, e.g., 10 and 50 liters/minute-m², while the flow rate of negative flow electrode 120 and positive flow electrode 160 can be on the lower end of this range, e.g., 1 and 10 liters/minute-m².

Method 400 proceeds with (block 430) applying the voltage between negative current collector 110 and positive current collector 170. In some examples, the applied voltage is between 1.5 V and 10 V per cell. In the same or other examples, the current density is between 1000 A/m² and 8000 A/m² (based on the membrane area).

Mainly in salt splitting electrolysis, an oxygen evolution reaction occurs (2H₂O → 4e⁻ + 4H⁺ + O₂) at the anode and a hydrogen evolution reaction (4H₂O → 4e⁻ + 4OH⁻ + 2H₂) occurs at the cathode. So the reactions usually don’t change and the salt ions never participate in the reactions. Because of the pH changes in the solution as a result of the water-splitting reactions, ions end up moving across the membranes to concentrate in different compartments.

Method 400 proceeds with (block 440) pumping negative flow electrode 120 into negative electrode recovery device 129 and (block 450) recovering negative flow electrode 120 using negative electrode recovery device 129. For example, negative active particles 122 of negative flow electrode 120 can be discharged using a ground connection within negative electrode recovery device 129 or by applying a reverse potential to negative flow electrode 120. Method 400 then proceeds with (block 460) pumping negative flow electrode 120 back to negative current collector 110.

In a similar manner, method 400 proceeds with (block 445) pumping positive flow electrode 160 into positive electrode recovery device 169 and (block 455) recovering positive flow electrode 160 using positive electrode recovery device 169. For example, positive active particles 162 of positive flow electrode 160 can be discharged using a ground connection within positive electrode recovery device 169 or by applying a reverse potential to positive flow electrode 160. Method 400 then proceeds with (block 465) pumping positive flow electrode 160 back to positive current collector 170.

Conclusion

Although the foregoing concepts have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing processes, systems, and apparatuses. Accordingly, the present embodiments are to be considered illustrative and not restrictive. 

1. A salt-splitting electrolysis system comprising: a feed stream, comprising a solvent and a salt, dissolved in the solvent and comprising cations and anions; a negative current collector; a negative flow electrode, comprising the solvent and negative active particles, suspended in the solvent and comprising a negative catalyst configured to react with the cations; a negative separating structure, disposed between and in contact with the feed stream and the negative flow electrode, wherein the negative separating structure is configured to selectively pass the cations from the feed stream to the negative flow electrode; a positive current collector; a positive flow electrode, comprising the solvent and positive active particles, suspended in the solvent and comprising a positive catalyst configured to react with the anions; and a positive separating structure, disposed between and in contact with the feed stream and the positive flow electrode, wherein the positive separating structure is configured to selectively pass the anions from the feed stream to the positive flow electrode.
 2. The salt-splitting electrolysis system of claim 1, wherein: each of the negative active particles comprises a negative base structure, comprising supporting the negative catalyst , and each of the positive active particles comprises a positive base structure, comprising supporting the positive catalyst.
 3. The salt-splitting electrolysis system of claim 2, wherein at least one of the negative base structure and the positive base structure comprises an electronically conductive material.
 4. The salt-splitting electrolysis system of claim 3, wherein the electronically conductive material is one or more of graphitic carbon, glassy carbon, graphene, carbon nanotubes, a transition metal carbide, iron oxide, platinum, and, iridium.
 5. The salt-splitting electrolysis system of claim 1, wherein: the negative catalyst comprises one or more of graphitic carbon, carbon-nitride, graphene, carbon nanotubes, transition metal carbides, platinum, palladium, rhodium, and iridium, and the positive catalyst comprises one or more of graphitic carbon, carbon-nitride, graphene, carbon nanotubes, a transition metal carbide, platinum, palladium, rhodium, iridium, metal oxide, and a perovskite oxide.
 6. The salt-splitting electrolysis system of claim 1, wherein: the negative separating structure is one of a cation-exchange membrane or a first size-exclusion porous separator, and the positive separating structure is one of an anion-exchange membrane or a second size-exclusion porous separator.
 7. A method of operating a salt-splitting electrolysis system, the method comprising: flowing a feed stream into a feed channel of the salt-splitting electrolysis system, wherein: the feed stream comprises cations and anions, the cations are selectively transferred to a negative flow electrode comprising negative active particles, the anions are selectively transferred to a positive flow electrode comprising positive active particles, the negative active particles comprise a negative catalyst configured to react with the cations in the feed stream; the positive active particles comprise a positive catalyst configured to react with the anions; and applying a voltage between a negative current collector and a positive current collector of the salt-splitting electrolysis system thereby causing the cations in the feed stream to react with the negative catalyst and further causing the anions in the feed stream to react with the positive catalyst.
 8. The method of claim 7, wherein: the negative active particles are suspended in a solvent and flow through a negative chamber of the salt-splitting electrolysis system, and the positive active particles are suspended in the solvent and flow through a positive chamber of the salt-splitting electrolysis system.
 9. The method of claim 8, wherein: the negative active particles are uniformly distributed through the negative flow electrode, and the positive active particles are uniformly distributed through the positive flow electrode.
 10. The method of claim 7, wherein: the negative active particles are contained within a set portion of a negative chamber of the salt-splitting electrolysis system such that a remaining portion of the negative flow electrode flows through the negative active particles, and the positive active particles are contained within a set portion of a positive chamber of the salt-splitting electrolysis system such that a remaining portion of the positive flow electrode flows through the positive active particles.
 11. The method of claim 7, further comprising: pumping at least a portion of the negative flow electrode into a negative electrode recovery device, recovering at least the portion of the negative flow electrode using the negative electrode recovery device, and pumping at least the portion of the negative flow electrode back to the negative current collector; and pumping at least a portion of the positive flow electrode into a positive electrode recovery device, recovering at least the portion of the positive flow electrode using the positive electrode recovery device, and pumping at least the portion of the positive flow electrode back to the positive current collector.
 12. The method of claim 11, wherein: recovering at least the portion of the negative flow electrode using the negative electrode recovery device comprises discharging the negative active particles of the negative flow electrode, and recovering at least the portion of the positive flow electrode using the positive electrode recovery device comprises discharging the positive active particles of the positive flow electrode.
 13. The method of claim 11, wherein: discharging the negative active particles of the negative flow electrode comprises flowing the negative active particles past a ground connection within the negative electrode recovery device or applying a reverse potential to the negative flow electrode, and discharging the positive active particles of the positive flow electrode comprises flowing the positive active particles past a ground connection within the positive electrode recovery device or applying a reverse potential to the positive flow electrode.
 14. The method of claim 7, wherein the voltage applied between the negative current collector and the positive current collector is between 1.5 V and 10 V per cell.
 15. The method of claim 7, wherein applying the voltage between the negative current collector and the positive current collector causes a current density between 2000 A/m²and 8000 A/m² (based on the membrane area) between the negative current collector and the positive current collector.
 16. The method of claim 7, further comprising pretreating the feed stream before flowing the feed stream into the feed channel.
 17. The method of claim 16, wherein pretreating the feed stream comprises flowing the feed stream through one or more ion-exchange columns, configured to selectively capture one or more of calcium (Ca), magnesium (Mg), iron (Fe), boron (B), sodium (Na), potassium (K), chloride (Cl), and sulfate (SO₄). 